This invention relates to a method for producing an integral titanium alloy article having at least two regions, each region having a distinct microstructure and mechanical properties.
The high strength-to-density ratio of titanium makes it a very attractive design choice in energy-efficient high thrust-to-weight gas turbine engines or airframes of modern airplanes. In titanium, the alloying elements tend to stabilize either the low-temperature close-packed hexagonal alpha phase, or the higher temperature allotrope, body-centered cubic beta phase. Titanium alloys for aerospace applications contain both alpha and beta stabilizing elements in various proportions depending on the application and, therefore, the required mechanical properties. The variety of compositions in titanium alloys arises in part because certain alloys are designed for optimization of certain properties. For example, for short-term strength, a relatively high beta stabilizer content is required. while for long-term creep strength, a relatively higher alpha stabilizer content is required.
The important high-temperature properties for aerospace related applications of titanium alloys are: tensile strength, creep, fatique initiation and fatigue crack propagation resistance, fracture toughness, hot salt stress and corrosion cracking, and oxidation resistance. In addition to selection of an alloy composition, processing of an alloy can be employed to provide desired properties.
In near-alpha and alpha+beta titanium alloys, the creep strength may be increased by heat treating or processing the material above the beta transus temperature. On cooling, this results in a lenticular alpha structure. This morphology has high creep strength, but has low low-cycle fatigue strength and low fatigue crack initiation resistance. On the other hand, an equiaxed alpha structure, such as resulting from sub-transus hot working and subsequent heat treatment, both in the alpha+beta phase field, exhibits lower creep resistance, but has high low-cycle fatigue strength and high fatigue crack initiation resistance. Thus, while it is possible to alter the microstructure of a titanium alloy article to obtain one or more desirable mechanical properties, such alteration often results in a diminution of one or more other mechanical properties. For example, a turbine rotor blade comprising an integral keyed configuration for mounting the blade within a matching slot on the periphery of a rotor may require creep resistance in the airfoil portion and high fatigue strength in the mounting portion. In such a turbine airfoil is heat treated to provide high creep resistance in the blade portion, it will likely have low fatigue strength in the mounting portion. Alternatively, if the blade is heat treated to provide high fatigue strength in the mounting portion, the airfoil portion will likely exhibit low creep strength.
Accordingly, it is an object of the present invention to provide a method for producing titanium alloy articles having both creep and low cycle fatigue resistance.
Other objects of the invention will be apparent to those skilled in the art form a reading of the following description of the invention.